Patent GB-226338-A (GB)
Title: Improvements in telephone and like electrical transmitting systems
IPC: H04B1/66
Filed: 1923-11-10
Abstract: 226,338. Western Electric Co., Ltd., (Western Electric Co., Inc.). Nov. 10, 1923. Thermionic generators; compounding frequencies; harmonics, proclucing.-In order that voice and like complex frequency current waves may be transmitted efficiently over a line, their frequencies are reduced before transmission and restored at the receiving end of the line. The unessential bands of frequencies in the voice range may be eliminated by filter circuits, and the essential bands reduced in frequency until they form a continuous band, which is then transmitted over a single line. Alternatively the voice range may be divided into narrow bands, each of which bands is reduced in frequency and transmitted over a separate circuit. Fig. 1 is a diagram illustrating the first system. Voice currents in the circuit 100 are transferred through a balanced transformer 103 and are divided by filters 105, 106, 107 into three essential frequency bands say 100-1000, 1500-2200, and 3600-3800. The lowest band is passed unchanged through the circuit 108 to the transmission line 102, the band 1500-2200 has its frequencies reduced to 1000-1700 by modulation at 113 with a frequency 500 supplied by the device 112, and the band 3600-3800 has its frequencies reduced to 1700-1900 by modulation at 116 with a frequency 1900 supplied by the device 115. A continuous band 100-1900 thus passes to the line 102 and to the station B. Here it enters the circuit 119 and is separated by band filters 120, 121 into the three bands 100-1000, 1000-1700, 1700-1900. The two latter bands are modulated at 301, 127 by waves of frequency 500 and 1900 respectively so as to raise the frequencies to 1500-2200, and 3600- 3800, and these bands together with the band 100-1000 pass jointly to the telephone line 101. The modulating waves at the station A are obtained by selecting harmonies from a harmonic producer 109 the basic frequency of which may be 50 cycles. This basic frequency is selected by a device 110 and is sent over the transmission line 102. At the receiving station the basic fre. quency is selected by a filter 123 and supplied to a harmonic producer 124 from which the modulating harmonics are selected at 302 and 126. Corresponding apparatus is employed for transmissions from station B to station A. The apparatus for reducing and increasing the frequencies may comprise push-pull valve modulators, which have the advantage of suppressing one of the input frequencies. The harmonic producer at one station may comprise a reactively-coupled valve and an amplifier, both provided with high resistances in their plate supply circuits. The basic frequency is sent out over a separate circuit and is used at the other station to overload a valve amplifier having high resistance in its plate supply circuit, so causing the generation of harmonics, which are utilized for raising the frequency of the various bands at that station. Fig. 7 shows a circuit arrangement for reducing an input frequency band of say 4000-6000 in the line 420 to an output band of 3000-5000 in the line 421, and vice versa for currents passing in the opposite direction. The frequency band in line 420 passes via the transformer 422 and filter 424 to a modulator 431 which is also supplied with a frequency of say 10000 from the device 434. The upper side band 14000-16000 is selected by the output filter 425 and is passed to a second modulator 431<1> where it is combined with a frequency 11000 supplied at 439. The lower side band 3000- 5000 is selected by the filter 441 and passed to the line 421. Currents of the band 3000-5000 in the opposite direction pass via filter 436, modulator 431', filter 426, modulator 431, filter 423 to line 420, being first raised in frequency to 14000-16000, and then reduced to 4000-6000. The two modulating frequencies 10000 and 11000 may be produced by modulating the mean frequency 10500 by half the difference 500, and isolating the two resulting frequencies by means of filters.